I. Field of the Invention
This invention relates to a process for producing nitrides of aluminum and titanium.
II. Description of the Prior Art
Aluminum and titanium nitrides are used in industry for a variety of purposes. For example, titanium nitride is used to coat metallic components, e.g. cutting tools, to improve their wear resistance, and aluminum nitride is currently being developed as an alternative to alumina in the formation of ceramic substrates for the electronics industry.
The most common method of forming metal nitrides is to react the corresponding metal with nitrogen. This method, however, requires the use of fine metal powders, which are very expensive, and it is not easy to control the particle size distribution of the product. Another method of forming nitrides is to react the corresponding metal oxide with carbon at high temperature in an atmosphere containing nitrogen. The main problem with such reactions is that a large proportion of the metal oxide and carbon often remain unreacted in the desired nitride product and are difficult to remove. This problem can be reduced if the oxide and carbon are finely ground and intimately mixed before the reaction is commenced, but at the reaction temperatures employed (usually 1300.degree.-2000.degree. C.), adjacent oxide particles may fuse together trapping carbon particles and making complete reaction quite difficult. Thus, the use of fine particles and intimate mixing cannot entirely overcome the problem of incomplete reaction of the starting materials.
Published Japanese patent application No. 1985-176910 filed in the name of Tokuyama Soda KK and laid open on Sep. 11, 1985 discloses a process for producing aluminum nitride powder in which alumina powder is mixed with a solution of an organic compound, such as polyvinyl alcohol, sucrose, starch etc., the solution is dried and the product heated to carbonize the organic compound, additional carbon is added and the resulting mixture is heated to high temperature under a nitrogen or ammonia atmosphere. While this process is claimed to produce finely divided AlN, attempts to duplicate this process have failed to produce a product of high purity. The particles are also reported to be significantly higher than 1 .mu. in size, whereas submicron powders are more preferable for ease of sintering and to obtain theoretical densities.
Published Japanese patent application No. 1980-15946 filed in the name of Toray Industries Co. and laid open for inspection on Feb. 4, 1980 discloses a method of forming a variety of metal nitrides, including AlN and TiN, from the corresponding oxides. In this process, a fine powder of the metal oxide is added to a solution of a carbohydrate, the solution is heated to dry it and to carbonize the carbohydrate, and the product is heated in a nitrogen-containing atmosphere. This process is similar to the one described above and does not yield the stated pure, finely divided product.
Accordingly, an object of the present invention is to provide a process for producing aluminum and titanium nitrides from the corresponding metal oxides which can result in a highly pure nitride product.
Another object of the invention is to provide such a process in which the size and shape of the resulting nitride particles can be controlled to some extent.